Postmitotical reconstruction of nucleoli in culturecells with UV-microbeam photoinactivated centrosome

Ultraviolet microirradiation of one of the poles of the mitotic spindle of PK cells was performed 1 min after the anaphase onset. Nucleolus formation in the telophase and G1 period were studied by vital observation, electron microscopy, and indirect immunofluorescense using antibodies against B23 protein. Sister cells with nonirradiated centrosomes and cells with partially irradiated cytoplasm were used as a control. During the first hour after the anaphase, nuclei in both sister cells were identical and contained numerous small dense particles with a granular ultrastructurc. B23 protein detected in the mitotic poles and at the chromosome surface in the anaphase was dispersed in the cytoplasm in both cells in the early G1 period. Later, control cells didn't display any difference from the intact cells: they formed nucleoli of a typical structure: B23 protein appeared in the karyoplasm and was then accumulated in nucleoli and disappeared from cytoplasm and karyoplasm. Nucleoli in cells with irradiated centrosomes did not achieve the normal size and contained significantly lower amount of the granular component. B23 protein was dispersed in the karyoplasm and was not accumulated in nucleoli. Nucleoli in cells with irradiated centrosomes contained small dense particles at least for 24 h. Telophase cells where microtubule formation had been inhibited by nocodazole formed normal nucleoli. It shows that the effects observed in cells with irradiated centrosomes are not due to the absense of the microtubule radial system. We conclude that UV microirradiation of the mitotic centrosome disturbs the postmitotic reconstruction of nucleoli probably because of the photodestruction of the B23 protein accumulated in the mitotic pole.